Keep in mind that a present perfect casts its predication in the present tense. It does not narrate past events, it mentions past events which give rise to a present state.

That is why formal English does not permit a present perfect to be used with a temporal adjunct which does not include the present moment, the Speech Time at which the sentence is uttered. (You will occasionally find this rule violated in improvised, conversational discourse; but even there it is comparatively rare.) Thus, this is acceptable:

okI have often visited London.

But this is not:

∗ I have often visited London in the 1990s.

Today, in the 20th century is a timeframe which excludes the present. Consequently:

Sentence 1) is acceptable if it appears in a text written in the present century, but would be of questionable acceptability if written fifteen years ago.

Sentence 2) is acceptable if it appeared in a text written during the 20th century, but it would not be acceptable if written today.

There is a great deal more about this at What is the perfect, and how should I use it?, especially §§ 3.1 Grammatical meaning, 3.2 Pragmatic meaning and 4. When and how should I use the perfect?.

∗ marks a usage as unacceptable